Explanation of error Cannot infer class of object

The XMI specification, clause 6.4.2, section 2g, states If the class of the object cannot
be determined unambiguously from the model, you must specify the class using thetype attribute.
In the situation being reported, xmi:type was not specified, so some inference
of what is intended is required.
However, the spec doesn't elaborate on how one should determine the class. The approach
taken by the Validator is to consider the declared type of the property of the object
(the object triggering the error report). If that type has subtypes, then there is apparently
no straightforward way to know which of those subtypes was intended. Thus the error report.

The error is identified during instantiation of objects from the XMI.
The error might be better classified as an XMI error, but it is not so classified now.